In small motors which employ a rotor coil or armature having a plurality of commutator segments, current is passed through the rotor coil through the use of brushes that receive current from a remote source such as a battery. The current to the coil is periodically reversed by the action of the commutator, thus maintaining the same direction of motor force within the motor. The brushes, which are typically made of carbon, are supported in a brush leaf in order to have the surface of the brush continuously make contact with the commutator segments during operation of the motor.
As the size of a motor decreases, it becomes more difficult to design and construct a brush leaf with the appropriate geometry so that the brush leaf will support a brush against the commutator segments with the correct pressure throughout the wear cycle of the brush. Further, it is also difficult to design a brush leaf assembly which provides a brush of sufficient length to meet the requirements of the life cycle of the motor.
There have been a number of designs for providing a brush leaf assembly in a motor having a housing diameter on the order of 34 millimeters. One of the more common solutions has been to design a brush leaf which acts as a cantilever spring to create a force for holding a brush mounted at the distal end of the spring in contact with the commutator. In order to ensure that the brush is in proper contact with the commutator, the cantilever spring should be of maximum length within the space available within the motor housing. In order to accomplish this, the prior art assemblies employ an angled leaf dividing the brush leaf into two sections formed on either side of the angle formed in the leaf. The second section of the leaf is used to connect the leaf to a terminal which in turn is connected to a source of current such as a battery.
FIGS. 18 and 19 show a prior art construction of a brush leaf for use in a small motor. In particular, FIG. 19 shows the use of a reinforced elbow 120 in order to try to eliminate the spring effect of the angle formed between the spring portion and the terminal portion of the copper strip. The complexity of the prior art system is also illustrated by the provision of a terminal portion 122 that is formed at an angle with a planar portion 124 which is also at an angle relative to the brush spring. As shown in FIG. 16, when the brush leaf is positioned within the motor, a plastic support 126 is provided within the motor housing to further reinforce the elbow 120 of the prior art brush leaf. At the same time, additional structure is needed to provide the terminals at the exterior of the motor. In particular, the terminal consists of a flat terminal portion 130 with a planar portion formed at an angle thereto, as can be seen in FIG. 17. The planar portion 132 contains an aperture which is positioned to mate with the aperture defined in the terminal 122. A securing grommet 136 is provided to secure the terminal 130 to the terminal portion 122 and the end cap of the motor.
Because of the angle between the section of the leaf used as a cantilever and the section used to connect to a terminal, the brush leaf becomes a compound spring in which the resilience of the angle makes some contribution to the force pressing the brush to the commutator. This extra resilience causes the brush leaf to behave unpredictably and, therefore, it is desirable to remove this extra resilience as a component of the spring force being exerted on the brush. One attempt to remove the unpredictable behavior has been to reinforce the elbow joint where the angle is formed in the brush leaf by providing a dimple to stiffen the joint so that it makes less of a contribution to the operation of the spring. Another solution has been to surround the elbow with a reinforcing plastic integral with the end cap of the motor into which the leaf and terminal are fitted. Although these prior art solutions have met with some success, there is nevertheless a need for an improved brush assembly which ensures that the brush pressure against the commutator segments on an armature is maintained constant throughout the life of the brush. The present invention is directed toward filling that need.